The present invention relates to a spread spectrum communication system and, more particularly, to a spread spectrum demodulating device for demodulating a spread spectrum signal by inverse spread.
In a spread spectrum communication signal, a spread spectrum signal is produced by spreading to N-fold a PSK (Phase Shift Keying) modulated data signal or PSK modulated signal by a PN (Pseudo Noise) signal. A device for receiving and demodulating the spread spectrum signal is generally constituted by a spread spectrum demodulating device and a PSK demodulating device. The spread spectrum demodulating device demodulates the PSK modulated signal by inversely spreading the received spread spectrum signal, while the PSK demodulating device regenerates the data signal by applying synchronization detection to the demodulated PSK modulated signal.
A prior art spread spectrum demodulating device which precedes a PSK demodulating device includes a mixer for inversely spreading an input spread spectrum signal to produce a demodulated PSK modulated signal. The demodulated PSK demodulated signal is passed through a band-pass filter to obtain a predetermined processing gain and then applied to a detector for envelop detection. The resulting output of the detector is fed to a comparator to determine whether or not a demodulated PSK demodulated signal has been detected. This kind of device, however, has a drawback that the decision associated with a demodulated PSK demodulated signal comes to involve errors and eventually results in malfunctions as the carrier-to-noise (C/N) ratio after the propagation of the signal through the band-pass filter approaches zero decibel. Further, each detector has a low-pass filter at its output stage and makes the band width of the filter extremely narrow under low C/N conditions so as to suppress the signal diffusion. This not only slows down the response but also results in a prohibitively long capture time. In addition, the prior art device is extremely complicated in construction.
For details of the spread spectrum demodulation techniques discussed above, a reference may be made to J. J. Spilker, Jr. "Delay-Lock Tracking of Binary Signals", IEEE TRANSACTIONS ON SPACE ELECTRONICS AND TELEMETRY 1963 March, page 7, FIG. 8.